Field of the Invention
The invention relates to an electronic circuit, and more particularly, to a current control circuit for a driving circuitry system of light-emitting diode (LED) components.
Description of the Related Art
Generally, an LED driving circuitry system is used to rectify an AC power voltage (VAC) from an electric grid for driving an LED component. A serious problem in this manner of driving is that there is a high level of total harmonic distortion (THD) in the circuit.
FIG. 1 shows the structure of a current three-stage LED driving circuitry system. A rectifier performs full-wave rectification on an AC power to generate a voltage Vo for driving the LED component at each stage. The waveform of the voltage Vo is a sine wave. With the increase of the voltage Vo, the LED component LED1 is turned on first, and, at the same time, the transistor M1 is turned on. At this time, the voltage Vcs at the common output terminal CS tracks the reference voltage VREF1. When the voltage Vo increases further, the LED component LED2 is turned on, and, at the same time, the transistor M2 is turned on. At this time, the voltage VCS at the common output terminal CS tracks the reference voltage VREF2, and the transistor M1 is turned off. When the voltage Vo continuously increases to a higher level, the LED component LED3 is turned on, and, at the same time, the transistor M3 is turned on. At this time, the voltage VCS at the common output terminal CS tracks the reference voltage VREF3, and the transistors M1 and M3 are turned off. In cases where the voltage Vo decreases, the above process is performed inversely.
In the circuit, the current IVo flowing through the LED component(s) is equal to the current ICS flowing through the common resistor RCS, represented by Equation (1):ICS=IVo=VCS/RCS  Equation (1)
VCS represents the voltage at the common output terminal CS. RCS represents the resistance value of the resistor RCS. VCS tracks the reference voltage VREF1, VREF2, or VREF3 when the transistor M1, M2, or M3 is turned on, respectively. The reference voltage VREF1, VREF2, and VREF3 are three basis voltages. The relationship between the reference voltages VREF1, VREF2, and VREF3 is VREF1<VREF2<VREF3. Thus, Equation (2) is obtained:ICS=IVo=VREF/RCS  Equation(2)
VREF is equal to the basis voltage VREF1, VREF2, or VREF3 with the variation of the turned-on states of the LED components at the respective stages.
Thus, with the increase of the voltage Vo, the waveform of the current ICS flowing through the common resistor RCS varies by a step phase. The current ICS is equal to VREF1/RCS, VREF2/RCS, or VREF3/RCS.
FIG. 2 is a is a schematic view showing the key voltages and currents of the three-stage LED driving circuitry system of FIG. 1. Since there is serious variation in the steps of the current Ics (IVo), the linearity of the circuitry system is low, and the THD is high. Currently, in order to enhance the linearity and lower the THD to approximately fit the envelope of the current ICS to the sine wave of the full-wave rectification (that is, being similar to the waveform of the voltage Vo), the number of stages of LED components has to increase, and the number of operational amplifier and transistors also has to increase, thereby increasing the number of steps of the current ICS and decreasing the amplitude of the steps. However, when the above manner is applied, the size of the driving circuitry system increases, and the cost of the driving circuitry system also increases seriously.